Electrode active material for lithium secondary battery

ABSTRACT

Provided is an electrode active material comprising a nickel-based lithium transition metal oxide (LiMO 2 ) wherein the nickel-based lithium transition metal oxide contains nickel (Ni) and at least one transition metal selected from the group consisting of manganese (Mn) and cobalt (Co), wherein the content of nickel is 50% or higher, based on the total weight of transition metals, and has a layered crystal structure and an average primary diameter of 3 μm or higher, wherein the amount of Ni 2+  taking the lithium site in the layered crystal structure is 5.0 atom % or less.

This application is a Divisional of application Ser. No. 12/792,374,filed Jun. 2, 2010, which claims the benefit of the filing date ofKorean Patent Application No. 10-2009-0049259, filed on Jun. 3, 2009, inthe Korean Intellectual Property Office, the disclosure of which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to an electrode active material forlithium secondary batteries. More specifically, the present inventionrelates to an electrode active material comprising a nickel-basedlithium transition metal oxide (LiMO₂) wherein the nickel-based lithiumtransition metal oxide contains nickel (Ni) and at least one transitionmetal selected from the group consisting of manganese (Mn) and cobalt(Co), wherein the content of nickel is 50% or higher, based on the totalweight of transition metals, and has a layered crystal structure and anaverage primary diameter of 3 μm or higher, wherein, for the layeredcrystal structure, the amount of Ni²⁺ taking the lithium site is 5.0atom % or less.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as energysources. Among these secondary batteries, lithium secondary batterieshaving high energy density and voltage, long cycle span and lowself-discharge are commercially available and widely used.

Meanwhile, the lithium secondary batteries generally use lithium cobaltcomposite oxide (LiCoO₂) as a cathode active material. Also, the use oflithium-manganese composite oxides such as LiMnO₂ having a layeredcrystal structure and LiMn₂O₄ having a spinel crystal structure andlithium nickel composite oxide (LiNiO₂) as the cathode active materialhas been considered.

Among these cathode active materials, LiCoO₂ is the most generally usedowing to superior physical properties such as superior cyclecharacteristics, but has low stability and is costly due to resourcelimitations of cobalt used as a raw material, thus disadvantageouslyhaving limited mass-utilization as power sources for electric vehicles.

Lithium manganese oxides such as LiMnO₂ and LiMn₂O₄ advantageously areredundant resources and use environmentally friendly manganese, thusattracting much attention as a cathode active material alternative toLiCoO₂. However, these lithium manganese oxides have disadvantages oflow capacity and bad cycle characteristics.

On the other hand, lithium nickel oxides such as LiNiO₂ are cheaper thanthe cobalt oxides and exhibit higher discharge capacity, when charged to4.3V and reversible capacity of doped LiNiO₂ reaches about 200 mAh/gwhich is higher than LiCoO₂ capacity (about 165 mAh/g). Accordingly, inspite of slightly low average discharge voltage and volumetric density,commercial batteries comprising LiNiO₂ as a cathode active material haveimproved energy density and a great deal of research on thesenickel-based cathode active materials is thus actively conducted inorder to develop high-capacity batteries. However, the problems ofLiNiO₂-based cathode active materials, including high preparation costs,swelling caused by gas generation in batteries, low chemical stabilityand high pH have not been solved yet.

In this regard, many conventional techniques focus on properties ofLiNiO₂-based cathode active materials and methods for preparing LiNiO₂and suggest lithium transition metal oxides wherein nickel is partiallysubstituted by other transition metals such as Co or Mn.

Meanwhile, lithium transition metal oxides used as cathode activematerials are in the form of a single particle (referred to as a“primary particle”) forming an independent structure, or of an aggregateparticle (referred to as a “secondary particle”) wherein two or moreprimary particles form an independent structure.

For such particle shapes, when an average particle diameter of primaryparticles (referred to as “average primary particle diameter”) is large,the press density of electrodes can be improved and capacity ofsecondary batteries can thus be considerably improved. In addition,variations in specific surface area with passage of time are decreased,making it easier to handle particles (in the form of a powder) andimproving processibility. Accordingly, a great deal of study to preparelithium transition metal oxides which can exert desired performancethrough prevention of deterioration in tap density or optimization ofparticle shapes such as preparation of spherical particles based oncontrol of factors such as particle size is underway.

However, in spite of these many advantages, nickel-based lithiumtransition metal oxides having an average primary particle diametercannot be applied to general batteries. This is the reason thatconventional techniques do not enable preparation of nickel-basedlithium transition metal oxides having a large average primary particlediameter and a completely-grown crystal structure, thus exhibitingdesired electrochemical performance.

In this regard, some related patents disclose primary particle diametersof Ni-, Mn- and Co-based lithium transition metal oxides.

For example, Japanese Patent Publication No. 2003-68299 disclosesLi—Mn—Ni—Co oxides having an average primary particle diameter of 3 μmor less. When the average diameter is higher than 3 μm, an electrolytesolution cannot be permeated into the particles. Preferably, the averagediameter is 1 μm or less. In addition, Comparative Example demonstratesthat, when the primary particle diameter is 2 μm or higher, batteryperformance, such as discharge capacity and cycle properties, isdeteriorated.

Korean Patent Laid-open No. 2008-0031424 discloses a cathode activematerial having a primary particle diameter of 0.1 μm to 3 μm. Inaddition, when the primary particle diameter is 3 μm or higher, a ratioof lithium ions which do not contribute to charge-discharge increases.Accordingly, it is preferred that the average diameter be 0.2 μm orless.

Also, many patents such as Japanese Patent Publication No. 2003-221236,Korean Patent Laid-open No. 2007-0097115 and Japanese Patent PublicationNo. 2008-84826 insist that an average primary particle diameter shouldbe small.

However, as mentioned below, as a result of a variety of extensive andintensive studies and experiments, and analysis and considerationassociated therewith, the inventors of the present invention confirmedthat, in the case where the average primary particle diameter is severalmicrometers (μm) or higher and a stable crystal structure is thusrealized, nickel-based active materials exhibiting superiorelectrochemical performance can be prepared.

Meanwhile, unlike the afore-mentioned patents, some related patentssuggest an active material having a relatively large primary particlediameter.

Specifically, Korean Patent Publication No. 2004-0106207 discloses acathode material wherein a plurality of primary particles are aggregatedto form a secondary particle, and a length at which adjacent primaryparticles are bound to each other on the cross-section of secondaryparticle is 10 to 70% with respect to the total circumference of thecross-section of primary particles. In this patent, the primary particlediameter is within the range of 0.2 to 10 μm. However, this patentteaches that Comparative Example is performed at a temperature which isunsuitable to realize relatively normal electrochemical performance andthe results thus obtained are compared with those of Example. When thesintering temperature is excessively low, the desired crystal structureis not formed and electrochemical performance of a material isdeteriorated.

Accordingly, this patent does not teach a cathode material having asubstantially large primary particle and exhibiting electrochemicalperformance.

Also, Japanese Patent Publication No. 2005-25975 suggests lithium nickelmanganese cobalt-based oxides which are represented byLi_(1+x)Ni_(1-y-z-p) Mn_(y)Co_(z)M_(p)O₂ (0≦x≦0.2, 0.1≦y≦0.5, 0.1≦z≦0.5,0≦z≦0.2, 0.2≦y+z+p≦0.8) and have an average primary particle diameter of3 to 20 μm. However, it is the most important that oxide particles havea large average primary particle diameter and superior crystallinity andthus exhibit superior electrochemical performance. The patent does notdisclose such a crystal structure and electrochemical performance. Inaddition, the method for preparing the crystal structure comprises amulti-step heating process such as heating under non-oxidativeatmosphere. The present invention enables preparation of a cathodematerial having a large and high average primary particle diameter andthus exhibiting superior electrochemical performance throughconventional single heating.

Also, Japanese Patent Publication No. 2006-54159 discloses a cathodeactive material for non-aqueous secondary batteries which containsnickel and lithium as main ingredients, is represented byLi_(x)Ni_(1-p-q-r)Co_(p)Al_(q)ArO_(2-y) (0.8≦x≦1.3, 0<p≦0.2, 0<q≦0.1,0≦r≦0.1, −0.3<y<0.1) and is composed of single crystals having anaverage particle diameter of 2 to 8 μm. However, this patent disclosesan active material which necessarily comprises inorganic chlorides orinorganic chloride oxides and teaches that, when an active material doesnot contain inorganic chlorides or inorganic chloride oxides or does notcontain a sufficient amount thereof, growth of primary particles isinhibited.

As mentioned above, regarding the primary particle size, the relatedpatents have different views. This is the reason that conventionalpatents focus on only the size of particles without considering thecrystal structure of materials.

Accordingly, there is an increasing need for electrode active materialsfor lithium secondary batteries having a large average primary particlediameter and a stable crystal structure, thus exhibiting goodelectrochemical performance.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention have discovered that an electrode active materialfor lithium secondary batteries having a stable crystal structure andthus exerting superior electrochemical performance can be prepared fromlithium transition metal oxide having a layered crystal structurewherein Ni content is 50% or higher, based on the total weight of alltransition metals, and having a predetermined average particle diameteror higher and a specific microstructure. The present invention wascompleted based on this discovery.

Technical Solution

Accordingly, the present invention provides an electrode active materialfor lithium secondary batteries composed of nickel-based lithiumtransition metal oxides (LiMO₂) which comprise nickel (Ni) and at leastone transition metal (M) selected from the group consisting of manganese(Mn) and cobalt (Co), wherein Ni is present in an amount of 50 mol %,based on the total mole number of all transition metals, and has alayered crystal structure and an average primary particle diameter of 3μm or higher, and the amount of Ni²⁺ taking the lithium site of thelayered crystal structure is lower than 5.0 atoms % or less.

As used herein, the term “average primary particle diameter” means notonly an average particle diameter of a primary particle as mentionedabove, but also an average particle diameter of a primary particle, forsecondary particles wherein a plurality of primary particles form anaggregate independent structure.

In accordance with the present invention, lithium transition metaloxides containing a large amount of nickel exhibit superiorelectrochemical performance although they have a larger primaryparticle, as compared to conventional lithium transition metal oxides.

As such, the fact that lithium transition metal oxides exhibit superiorelectrochemical properties in spite of large average primary particlediameter of 3 μm or higher is due to high crystallinity based on themicrostructure.

That is, for the layered crystal structure, the amount of Ni²⁺ takingthe lithium site is 5.0 atom % or less and a more detailed explanationthereof is mentioned below.

For nickel-based lithium transition metal oxides having a layeredstructure of the present invention, the size difference of cations is anessential factor to form a crystal structure. Specifically, fornickel-based lithium transition metal oxides, as the size differencebetween the lithium ion and the transition metal ion increases, amaterial having a complete and high crystallinity structure can beprepared.

For lithium transition metal oxides, comparing cation sizes, the sizedifference between Ni³⁺, Co³⁺ or Mn⁴⁺, and Li⁺ is great and thepossibility of cation mixing occurring is thus considerably low. On theother hand, among nickel ions, nickel bivalent cations (Ni²⁺) have alarge size, comparable to lithium ions, thus causing the cation mixingbehaviors. When Ni²⁺ is mixed with Li⁺ and is incorporated in thelithium layer, the crystallinity of ion structures is deteriorated, thuscausing deterioration in battery performance.

FIGS. 1, 2 and 3 are TG graphs showing weight decrease as a function ofelevating temperature, for a nickel salt, a manganese salt and a cobaltsalt, respectively. As can be seen from the graphs, since nickel isconsiderably stable in the +2 state unlike manganese or cobalt, for asubstance containing a great amount of nickel, the amount of Ni²⁺increases.

That is to say, a material containing a great amount of nickel, morespecifically, a material containing nickel in an amount of 50 mol % orhigher, based on the total content of transition metals, contains agreat amount of Ni²⁺. Such Ni²⁺ is larger than other ions and iscomparable in size to a lithium ion, and may be thus mixed in thelithium layer. Ni²⁺ incorporated into the lithium layer inhibitstransfer of lithium ions in nickel particles, thus causing deteriorationin battery performance. This problem always occurs in lithium transitionmetal oxides containing a great amount of nickel, in particular, becomesmore serious in materials containing a great amount of nickel.

On the other hand, for nickel-based lithium transition metal oxides, theamount of Ni²⁺ taking lithium sites in a layered crystal structure is5.0 atom % or less, thus efficiently inhibiting incorporation of Ni²⁺into the lithium layer and exhibiting superior crystallinity. As aresult, the nickel-based lithium transition metal oxides exhibitsuperior electrochemical performance due to their more stable layeredstructure. More preferably, Ni²⁺ taking the lithium sites may be presentin an amount of 0.1 to 4.0 atom %.

The nickel-based lithium transition metal oxides according to thepresent invention comprise nickel (Ni) in an excessive amount of 50% orhigher, based on the total mole number of transition metals, thusexerting high capacity. Preferably, the content of Ni is 50% to 90%.When the content of Ni is less than 50%, high capacity cannot berealized. On the other hand, when the content of Ni exceeds 90%,impurities are increased and high-temperature stability is thusconsiderably deteriorated due to high reactivity with an electrolytesolution.

As mentioned above, the nickel-based lithium transition metal oxidecontains nickel (Ni) and at least one transition metal selected from thegroup consisting of manganese (Mn) and cobalt (Co). Preferably, thenickel-based lithium transition metal oxide contains both manganese (Mn)and cobalt (Co). In this case, Mn is present in an amount of 10% to 45%and Co is present in an amount of 5% to 40%, based on the total molenumber of transition metals.

In this case, the transition metals may be partially substituted by oneor more elements selected from the group consisting of Al, Mg, Ti andZr.

The amount of elements substituted is 0.1 to 5%, based on the total molenumber of transition metals. When the content is less than 0.1%, theeffects obtained by substitution cannot be realized. On the other hand,when the content exceeds 5%, the amount of transition metals such asnickel is decreased and battery capacity is disadvantageously thusdecreased.

As mentioned above, the nickel-based lithium transition metal oxide ofthe present invention has a high-crystallinity structure and an averageprimary particle diameter of 3 μm or higher, more preferably, 3 to 10μm.

The electrode active material comprising nickel-based lithium transitionmetal oxides may have a primary particle structure composed of singleparticles with an average particle diameter of 3 μm or higher, or asecondary particle structure which is composed of an aggregate of thesingle particles and has inner pores. Such an aggregate-type particlestructure maximizes the surface area at which the particles react withan electrolyte solution, thereby exerting high rate characteristics andimproving reversible cathode capacity.

In a preferred embodiment, for the nickel-based lithium transition metaloxide of the present invention, a ratio of a transition metal (M) tolithium is 1.005 to 1.30, based on the mole number, more preferably,1.01 to 1.20. As mentioned below, this ratio is important to form aspecific crystal microstructure.

Also, the present invention provides a lithium secondary batterycomprising the electrode active material as a cathode active material.The lithium secondary battery, for example, comprises a cathode, ananode, a separator, and a lithium salt-containing non-aqueouselectrolyte.

For example, the cathode is prepared by applying a cathode mixcomprising a cathode active material, a conductive material, a binderand a filler to a cathode current collector, followed by drying. Thecathode mix may comprise the afore-mentioned ingredients, i.e., theconductive material, the binder and the filler.

As examples of the anode active material that can be used in the presentinvention, mention may be made of carbon and graphite materials such asnatural graphite, artificial graphite, expanded graphite, carbon fibers,non-graphitizing carbon, carbon black, carbon nanotubes, fullerenes andactivated carbon; metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb,Pd, Pt and Ti, which are alloyable with lithium metal, and compoundscontaining such elements; a composite material of a metal or metalcompound and a carbon or graphite material; and lithium nitride. Amongthese, more preferred are the carbon-, silicon-, tin-,silicon/carbon-based active materials, which may be used alone or in anycombination thereof.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

Examples of binders that may be used in the present invention includepolytetrafluoroethylene (PTFE), polyvinyl alcohols,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymers(EPDM), sulfonated EPDM, styrene butadiene rubbers, fluoro rubbers,various copolymers, highly-saponificated polyvinyl alcohols and thelike.

The conductive material improves conductivity of the electrode activematerial and is commonly added in an amount of 1 to 30% by weight, basedon the total weight of the electrode mix. Any conductive material may beused without particular limitation so long as it has suitableconductivity without causing adverse chemical changes in the fabricatedsecondary battery. As examples of the conductive materials that can beused in the present invention, mention may be made of conductivematerials, including graphite such as natural or artificial graphite;carbon blacks such as carbon black, acetylene black, Ketjen black,channel black, furnace black, lamp black and thermal black; conductivefibers such as carbon fibers and metallic fibers; metallic powders suchas carbon fluoride powders, aluminum powders and nickel powders;conductive whiskers such as zinc oxide and potassium titanate;conductive metal oxides such as titanium oxide; and polyphenylenederivatives.

The viscosity controlling agent is an ingredient to control theviscosity of the electrode mix and thereby easily mix the electrode mixand apply the same to a current collector, and is present in an amountof 30 wt % or less, based on the total weight of the electrode mix.Examples of viscosity controlling agents include, but are not limitedto, carboxymethyl cellulose and polyvinylidene fluoride. In some cases,the afore-mentioned solvent may act as the viscosity controlling agent.

The filler is a component used to inhibit expansion of the electrode.There is no particular limit to the filler, so long as it does not causeadverse chemical changes in the fabricated battery and is a fibrousmaterial. As examples of the filler, there may be used olefin polymerssuch as polyethylene and polypropylene; and fibrous materials such asglass fibers and carbon fibers.

The coupling agent is an auxiliary ingredient to improve the adhesionbetween electrode active materials and the binder, has two or morefunctional groups and may be used in an amount of 30% by weight, basedon the weight of the binder. For example, the coupling agent may be amaterial which has one functional group which reacts with a hydroxylgroup or a carboxyl group present on the surface of silicon-, tin-,graphite-based active materials to form a chemical bond and otherfunctional groups which react with a polymeric binder to form a chemicalbond. Specifically, examples of coupling agents include, but are notlimited to, silane-based compounds such as triethoxysilylpropyltetrasulfide, mercaptopropyl triethoxysilane, aminopropyltriethoxysilane, chloropropyl triethoxysilane, vinyl triethoxysilane,methacryloxypropyl triethoxysilane, glycidoxypropyl triethoxysilane,isocyanatopropyl triethoxysilane, and cyanatopropyl triethoxysilane.

The adhesion promoter is an auxiliary ingredient to improve adhesion ofan active material to a current collector, is present in an amount of 10wt %, based on the binder and examples thereof include oxalic acid,adipic acid, formic acid, acrylic acid derivatives and itaconic acidderivatives.

Examples of molecular weight controllers that can be used in the presentinvention include t-dodecyl mercaptan, n-dodecyl mercaptan andn-octylmercaptan. Examples of cross-linking agents include1,3-butanediol diacrylate, 1,3-butanediol dimethacrylate, 1,4-butanedioldiacrylate, 1,4-butanediol dimethacrylate, aryl acrylate, arylmethacrylate, trimethylolpropane triacrylate, tetraethylene glycoldiacrylate, tetraethylene glycol dimethacrylate and divinylbenzene.

The current collector is a site where electrons are transferred duringelectrochemical reactions of active materials. Depending on the type ofthe electrode, the current collector is divided into an anode currentcollector and a cathode current collector.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to the anode currentcollector, so long as it has suitable conductivity without causingadverse chemical changes in the fabricated battery. As examples of theanode current collector, mention may be made of copper, stainless steel,aluminum, nickel, titanium, sintered carbon, and copper or stainlesssteel surface-treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to the cathodecurrent collector, so long as it has suitable conductivity withoutcausing adverse chemical changes in the fabricated battery. As examplesof the cathode current collector, mention may be made of stainlesssteel, aluminum, nickel, titanium, sintered carbon, and aluminum orstainless steel surface-treated with carbon, nickel, titanium, silver orthe like.

If necessary, these current collectors may also be processed to formfine irregularities on the surface thereof so as to enhance adhesion tothe cathode active materials. In addition, the current collectors may beused in various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

The lithium salt-containing, non-aqueous electrolyte is composed of anon-aqueous electrolyte and a lithium salt.

As the non-aqueous electrolytic solution that can be used in the presentinvention, for example, mention may be made of non-protic organicsolvents such as N-methyl-2-pyrollidinone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and may include, for example,LiC1, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imide.

In some cases, an organic solid electrolyte or an inorganic solidelectrolyte may be used.

As examples of the organic solid electrolyte utilized in the presentinvention, mention may be made of polyethylene derivatives, polyethyleneoxide derivatives, polypropylene oxide derivatives, phosphoric acidester polymers, poly agitation lysine, polyester sulfide, polyvinylalcohols, polyvinylidene fluoride, and polymers containing ionicdissociation groups.

As examples of the inorganic solid electrolyte utilized in the presentinvention, mention may be made of nitrides, halides and sulfates oflithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄,LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally include carbon dioxide gas.

The lithium secondary batteries of the present invention may befabricated by a conventional method known in the art. Also, the cathode,anode and separator structures of the lithium secondary batteries arenot particularly limited and, for example, respective sheets areinserted into a cylindrical, square or pouch case in a winding- orstacking-type.

In addition, the present invention provides a method for preparing theelectrode active material comprising mixing a lithium-containingprecursor with a transition metal-containing precursor under thecondition that a molar ratio of a transition metal to lithium is higherthan 1 and heating the resulting mixture at a higher temperature.

When the heating temperature is excessively low, raw materials remain inthe particles due to insufficient reactions, high-temperature stabilityof batteries may be damaged, and volumetric density and crystallinityare deteriorated, thus making it difficult to maintain the stable shape.On the other hand, when the heating temperature is excessively high,particles may be non-uniformly grown, oxygen deintercalation of thestructure occurs, and electrochemical performance of a material is alsodeteriorated.

Accordingly, the heating temperature is higher (10° C. or higher) than aheating temperature at which lithium transition metal oxide containingan equivalent amount of lithium and a transition metal (that is, lithiumtransition metal oxide having a layered crystal structure, wherein a mixratio of a transition metal to lithium is 1), is prepared from a mixtureof a lithium-containing precursor and a transition metal-containingprecursor, and is lower than an oxygen deintercalation temperature ofthe lithium transition metal oxide.

Generally, nickel-based lithium transition metal oxide has a suitablesintering temperature, depending on the composition of transitionmetals. This is the reason that the sintering temperature is closelyrelated to development of crystal structure.

When the sintering temperature is lower than the desired level, thecrystal structure of a material is insufficient, thus causingdeterioration in electrochemical performance. In addition, a sufficientenergy is preferably supplied to efficiently develop a crystal structurefor material growth. Accordingly, it is preferred that the sinteringtemperature is sufficiently high to grow a crystal structure.

However, when the sintering temperature is excessively high, out of thedesired range, oxygen deintercalation in the structure occurs and thestructure is deformed. FIG. 4 is a TG graph showing weight variation ofa mixture of a transition metal salt and a lithium salt, with heating.When the temperature is higher than a predetermined level, all reactionsare completed, the weight does not decrease, and the state is maintainedalthough the temperature increases. However, when the temperature isincreased to a higher level, oxygen is deintercalated and, at the sametime, weight is decreased.

When oxygen is deintercalated, Ni²⁺ having a lower oxidation number isgenerated to maintain electric balance, thus deforming the layeredstructure. Accordingly, sintering of nickel-based lithium transitionmetal oxides is generally performed at a predetermined sinteringtemperature. As mentioned above, this is the reason that when thesintering temperature is out of a general range, the crystal structureof a material cannot form a stable layered structure.

Meanwhile, the average primary particle diameter of nickel-based lithiumtransition metal oxide is also related to sintering conditions. When anickel-based compound is prepared while elevating the sinteringtemperature, as the sintering temperature increases, the average primaryparticle diameter of particles gradually increases (See ComparativeExample, Table 5). Accordingly, when the sintering temperature isincreased to a considerably high level, nickel-based lithium transitionmetal oxide having a large average primary particle diameter can beobtained. When the average primary particle diameter increases, variousadvantages including an increase in press density of a material can beobtained.

However, when the sintering temperature is elevated to a higher level inorder to increase an average primary particle diameter, oxygen isdeintercalated and Ni²⁺ is thus generated. For this reason, Ni²⁺ isincorporated into the lithium layer, thus deforming the crystalstructure and causing deterioration in electrochemical performance.

Accordingly, it was known that it is important to prepare nickel-basedlithium transition metal oxide having a large average primary particlediameter and a stable structure, thus exhibiting superiorelectrochemical performance. For this reason, synthesis is generallyperformed at a sintering temperature which is not excessively high andis not excessively low and conventional nickel-based lithium transitionmetal oxides have a small average primary particle diameter of 0.1 to 1μm.

In this regard, nickel-based lithium transition metal oxide wherein aratio of a transition metal to lithium is higher than 1, morepreferably, 1.005 to 1.30 is prepared.

The afore-mentioned reaction conditions enable preparation of a crystalstructure having superior crystallinity. Considering the fact thatlithium and a transition metal take respective sites, as the amount oflithium ions increases, a ratio at which the lithium ions take thelithium site increases. In addition, an oxidation number of thetransition metal, in particular, nickel (Ni), in lithium transitionmetal oxides increases. That is, Ni²⁺ is converted into Ni³⁺. As theoxidation number of transition metals increases, the bonding energybetween the transition metal ion and the oxygen ion increases and thedistance between transition metal layers narrows. As a result, thedistance between the lithium layer and the transition metal layerincreases. Accordingly, lithium ions having a large size are notincorporated into the transition metal layer and remain in the lithiumlayer. As a result, the crystal structure having superior crystallinityis realized, thus promoting movement of lithium in the structure andimproving electrochemical performance of batteries.

However, when an excessive amount of lithium is added, lithium residuessuch as Li₂CO₃ or LiOH remain on the particle surface, thus adverselyaffecting processes required for battery fabrication and batteryperformance. Conventional Ni, Mn, and Co-based lithium transition metaloxides exhibiting superior electrochemical performance are composed ofsecondary particles wherein primary particles having an average particlediameter of about 1 μm or less are aggregated. Accordingly, a surfacearea in which particles contact the outside increases, a great amount ofreaction residues are generated due to the side reaction with anelectrolyte, when finally fabricated batteries are operated, andproblems such as swelling occur.

On the other hand, as mentioned above, when synthesis is simplyperformed using an excess of lithium under conventional sinteringtemperature conditions, more lithium residues remain, as compared to thecase where lithium in a mole number equivalent to nickel is sintered.Accordingly, as mentioned above, although crystallinity improvement canbe realizes when an excess of lithium is added, Ni, Mn and Co-basedlithium transition metal oxides entail the problem such as swelling dueto lithium residues and simple addition of excess lithium is thusunsuitable.

Consequently, to synthesize nickel-based lithium transition metal oxideshaving a large average primary particle diameter and a crystal structurewith superior crystallinity, thus exhibiting superior electrochemicalperformance, sintering should be carried out at a temperature higherthan a general sintering temperature and a ratio of a transition metalto lithium should be higher than 1.

As mentioned above, when the sintering temperature is simply increased,the problem, incorporation of Ni²⁺ into the lithium layer, occurs, butexcess lithium is added to increase the possibility in which lithiumions take the lithium site, and larger Ni²⁺ is converted into smallerNi³⁺ to inhibit incorporation of nickel ions into the lithium layer.Accordingly, lithium transition metal oxide which has an average primaryparticle diameter of 3 μm or higher and exhibits superiorelectrochemical performance can be obtained. In addition, as the averageprimary particle diameter increases, a specific surface area at whichparticles contact the outside decreases, thus reducing side reactionswith the electrolyte. That is, when lithium transition metal oxides aresynthesized using the method according to the present invention, theproblem, an increase in lithium residues caused by addition of excesslithium, can be solved by increasing the average primary particlediameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1, 2 and 3 are TG graphs showing weight decrease as a function ofelevating temperature, for a nickel salt, a manganese salt and a cobaltsalt, respectively;

FIG. 4 is a TG graph showing weight variation of a mixture of atransition metal salt and a lithium salt, with heating;

FIG. 5 is an SEM image showing lithium transition metal oxide preparedin Example 1;

FIG. 6 is an SEM image showing lithium transition metal oxide preparedin Example 2;

FIG. 7 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 2;

FIG. 8 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 4;

FIG. 9 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 5;

FIG. 10 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 6;

FIG. 11 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 7;

FIG. 12 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 8; and

FIG. 13 is an SEM image showing lithium transition metal oxide preparedin Comparative Example 9.

BEST MODE

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

Ni, Mn and Co salts (hereinafter, referred to as metal hydroxides) wereadded in molar ratios of the transition metals mentioned below forelectrode active materials to distilled water and thoroughly mixed, toprepare a precipitate while increasing pH. The precipitate thus preparedwas washed, filtered using a filter paper and dried in an oven at 130°C. overnight to obtain a powder. The powder was mixed with a Li salt inan amount, enabling that the amount of Ni²⁺ taking all lithium sites ofthe resulting lithium transition metal oxide is 5.0 atom % or less, andthe mixture was heated in an electric furnace at 940° C. for 12 hourssuch that the average primary particle diameter was 3 μm or higher toprepare an electrode active material (See FIG. 5). For the electrodeactive materials thus prepared, molar ratios of transition metals, Ni,Mn and Co were 67%, 13% and 20%, respectively.

Comparative Example 1

An electrode active material was prepared in the same manner as Example1, except that the molar number of transition metal in the transitionmetal salt was equivalent to that of lithium in the lithium salt, andthe mixed powder was heated in an electric furnace at 940° C. for 12hours such that the average primary particle diameter was 3 μm orhigher.

Comparative Example 2

An electrode active material was prepared in the same manner asComparative Example 1, except that the mixed powder was heated at 865°C. such that primary particles having a small particle diameter areaggregated to form a secondary particle (See FIG. 7).

Example 2

An electrode active material was prepared in the same manner as Example1, except that metal hydroxide wherein the molar ratios of transitionmetals, Ni, Mn and Co were 53%, 27% and 20%, respectively, was used andthe heating temperature was 970° C. (See FIG. 6).

Comparative Example 3

An electrode active material was prepared in the same manner as Example2, except that the molar number of transition metal in the transitionmetal salt was equivalent to that of lithium in the lithium salt and themixed powder was heated in an electric furnace at 970° C. for 12 hourssuch that the average primary particle diameter was 3 μm or higher.

Comparative Example 4

An electrode active material was prepared in the same manner asComparative Example 3, except that the mixed powder was heated at 925°C. such that primary particles having a small particle diameter areaggregated to form a secondary particle (See FIG. 8).

Comparative Examples 5 to 9

An electrode active material was prepared in the same manner asComparative Example 2, except that the heating temperature was 700 to910° C. (See FIGS. 9 to 13)

Experimental Example 1

The powders prepared in Examples 1 and 2, and Comparative Examples 1 to9 were filled with a resin and fixed thereon, and the cross-section ofthe particles cut such that the cross-section of them was exposed wasobserved by SEM. 50 particles were randomly selected, the long and shortaxis lengths thereof were measured and the average length thereof wascalculated, to obtain an average primary particle diameter. The resultsthus obtained are shown in Table 1 below.

Experimental Example 2

Each electrode active material prepared in Examples 1 and 2, andComparative Examples 1 to 9, Denka Black as a conductive material andPVdF as a binder were mixed in a ratio of 95%, 2.5% and 2.5%,respectively, NMP was added to the mixture and the resulting mixture wasthoroughly mixed to prepare a slurry with a suitable viscosity. Theslurry was applied to an aluminum foil with a thickness of 20 μm, driedat 130° C. and roll-pressed to fabricate a battery.

A lithium foil acting as an anode, a separator and an electrolytesolution were assembled to fabricate a coin battery. The battery wassubjected to electrochemical experiments. The first cycle dischargecapacity was measured to obtain a discharge capacity and the resultsthus obtained are shown in Table 1 below.

Experimental Example 3

Ni occupancy refers to a ratio of Ni²⁺ taking all lithium sites, andRietveld refinement for X-ray diffraction patterns was performed inorder to measure the Ni occupancy of electrode active materials preparedin Examples 1 and 2 and Comparative Examples 1 to 9. The results thusobtained are shown in Table 1 below.

Experimental Example 4

Electrode active material slurries prepared in Examples 1 and 2 andComparative Examples 2 and 4 were applied to an aluminum foil to a totalthickness of 200 μm and dried at 130° C. for 20 minutes to fabricate anelectrode. The electrode thus dried was cut to a predetermined size androll-pressed 3 times under a predetermined pressure. The thickness ofthe resulting product was measured and a thickness decrease forrespective measurements was calculated. The results thus obtained areshown in Table 2 below.

TABLE 1 Average primary particle Heating Discharge diameter temperaturecapacity Ni Occupancy (μm) (° C.) (mAh/g) (atom %) Ex. 1 5.65 940 176.72.03 Ex. 2 4.98 970 162.5 1.93 Comp. Ex. 1 5.46 940 160.4 5.19 Comp. Ex.2 0.88 865 177.6 2.07 Comp. Ex. 3 5.02 970 155.4 5.05 Comp. Ex. 4 0.95925 162.7 1.96 Comp. Ex. 5 0.14 700 130.5 7.63 Comp. Ex. 6 0.25 760155.3 5.12 Comp. Ex. 7 0.58 850 177.1 2.52 Comp. Ex. 8 1.43 880 176.92.11 Comp. Ex. 9 2.95 910 170.7 3.02

As can be seen from Table 1 above, a predetermined sintering temperatureat which electrochemical performance is superior exists, and whensintering is performed at a temperature higher than this temperature, alarge average primary particle diameter can be realized, but structuraldefects occur, thus causing deterioration in electrochemicalperformance.

TABLE 2 Electrode thickness decrease (μm) 1^(st) 2^(nd) 3^(rd) Ex. 1123.8 12.7 3.1 Ex. 2 124.4 12.6 3.3 Comp. Ex. 2 123.9 7.5 0.5 Comp. Ex.4 123.9 7.7 0.4

In addition, as can be seen from Table 2 above, materials having a largeaverage primary particle diameter are readily rolled, improvingprocessibility, decreasing the thickness of the final products, andimproving space efficiency in the process of fabricating batteries,thereby contributing to improvement of battery capacity.

INDUSTRIAL APPLICABILITY

As apparent from the afore-going, the electrode active material forlithium secondary batteries comprises 50% or higher of nickel, based onthe total weight of transition metals, has a layered crystal structureand has an average primary particle diameter of 3 μm or higher. Thelithium secondary batteries fabricated from the electrode activematerial due to the large average primary particle diameter and stablestructure exert superior electrochemical performance and advantageouslysolve problems such as slurry gelation and deterioration in batteryhigh-temperature performance due to considerably decreased byproducts.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method for preparing an electrode activematerial comprising: mixing a lithium-containing precursor with atransition metal-containing precursor under the condition that a molarratio of a transition metal to lithium is higher than 1; and heating theresulting mixture at a higher temperature, wherein the heatingtemperature is 10° C. or higher than a heating temperature at whichlithium transition metal oxide having a layered crystal structure isprepared from a mixture of a lithium-containing precursor and atransition metal-containing precursor under the condition that the ratioof the transition metal to lithium is 1, and is lower than an oxygendeintercalation temperature of the lithium transition metal oxide,wherein the electrode active material comprises a nickel-based lithiumtransition metal oxide containing nickel (Ni) and at least onetransition metal selected from the group consisting of manganese (Mn)and cobalt (Co), wherein a content of nickel is 50% or higher, based onthe total weight of transition metals, and wherein the nickel-basedlithium transition metal oxide has a layered crystal structure and anaverage primary diameter of 3 μm or higher.
 2. The method according toclaim 1, wherein the ratio of the transition metal to lithium is 1.005to 1.30.
 3. The method according to claim 1, wherein the amount of Ni²⁺taking the lithium site of the layered crystal structure is 5.0 atom %or less.
 4. The method according to claim 1, wherein the amount of Ni²⁺taking the lithium site of the layered crystal structure is 0.1 to 5.0atom %.
 5. The method according to claim 1, wherein the amount of Ni²⁺taking the lithium site of the layered crystal structure is 0.1 to 4.0atom %.
 6. The method according to claim 1, wherein the nickel-basedlithium transition metal oxide an average primary diameter of 3 μm to 10μm.